A charge-transporting polymer represented by polyvinyl carbazole (PVK) is useful as a photoconductive material for electrophotographic photoreceptor or an organic electric field light-emitting element material as described in transactions of the 37th Joint Seminary of Applied Physics, 31p-K-12 (1990). In both of the above cases, such a charge-transporting polymer is formed into a layer which is used as a charge-transporting layer. A charge-transporting polymer such as PVK and a low molecular weight dispersion system having a low molecular charge-transporting material dispersed in a polymer are well known as the materials for forming the charge transporting layer. An element having a low molecular weight charge-transporting material vacuum-evaporated thereon is generally used as the organic electric field light-emitting element. Among these materials, the low molecular dispersion system is major in the electrophotographic photoreceptor because it has a wide diversity and can easily provide a high functional product. In recent years, organic photoreceptors have been more and more used in high speed copying machines or printers as their performance has been enhanced. However, the ability of these organic photoreceptors may often insufficient even now for use in high speed copying machines or printers. In particular, it has been keenly desired to further prolong the life of these organic photoreceptors. One of the important factors determining the life of these organic photoreceptors is abrasion of the charge-transporting layer. The charge-transporting layer comprising a low molecular weight dispersion system, which is major at present, has been more and more satisfactory with respect to electrical properties. However, such a charge-transporting layer is disadvantageous in that it is essentially apt to mechanical abrasion because of its structure that comprises a low molecular weight compound dispersed in a polymer. Further, the organic electric field light-emitting element is disadvantageous in that the accompanying Joule's heat causes melting of the low molecular weight charge-transporting material, followed by morphological change of the film due to crystallization or the like.
On the other hand, a charge-transporting polymer has a possibility for drastically eliminating these disadvantages and thus is extensively studied. For example, U.S. Pat. No. 4,806,443 discloses a polycarbonate obtained by the polymerization of specific dihydroxyarylamine and bischloroformate. U.S. Pat. No. 4,806,444 discloses a polycarbonate obtained by the polymerization of specific dihydroxyarylamine and phosgene. U.S. Pat. No. 4,801,517 discloses a polycarbonate obtained by the polymerization of bishydroxyalkylarylamine and bischloroformate or phosgene. U.S. Pat. Nos. 4,937,165 and 4,959,288 disclose a polycarbonate obtained by the polymerization of specific dihydroxyarylamine or bishydroxyalkylarylamine and bischloroformate or a polyester obtained by the polymerization of specific dihydroxyarylamine or bishydroxyalkylarylamine and bisacyl halide. Further, U.S. Pat. No. 5,034,296 discloses a polycarbonate or polyester of arylamine having a specific fluorene skeleton. U.S. Pat. No. 4,983,482 discloses a polyurethane. Still further, JP-B-59-28903 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses a polyester having a specific bisstyrylbisarylamine as a main chain. Further, JP-A-61-20953 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-1-134456, JP-A-1-134457, JP-A-1-134462, JP-A-4-133065, and JP-A-4-133066 propose a polymer having a charge-transporting substituent such as hydrazone and triarylamine as a pendant and a photoreceptor comprising such a polymer.
A charge-transporting polymer is required to exhibit various properties such as solubility, mobility and matching in oxidation potential. In order to satisfy these requirements, it is commonly practiced to introduce various substituents into the charge-transporting polymer to control the physical properties thereof. The ionization potential of a charge-transporting polymer is almost determined by the charge-transporting monomers constituting the charge-transporting polymer. Thus, it is important that the ionization potential of the charge-transporting monomers be controllable. The monomers as starting materials of the above described triarylamine polymer can be roughly divided into two types, i.e., (1) monomers having two hydroxyphenyl groups and (2) monomers having two hydroxyalkylphenyl groups. However, the monomers having two hydroxyphenyl groups easily becomes aminophenol structure and thus can be easily oxidized and can hardly be purified. Further, monomers having a parahydroxy structure is more unstable. It is difficult to control the ionization potential of these monomer by changing the position of substituents. Further, since these monomers have an aromatic ring directly substituted by oxygen, its electron attractive property easily causes biased electric charge distribution, to thereby easily deteriorate its mobility. The monomers having two hydroxyalkylphenyl groups has no adverse effects due to electron attractive property of oxygen because of interposition of a methylene group. However, these monomers can be hardly synthesized. That is, the reaction of diarylamine or diarylbenzidine with 3-bromoiodobenzene tends to provide a mixed product because both bromine and iodine are reactive. Thus, the yield is reduced. Further, an alkyl lithium which is used in lithiumation of bromine, and ethylene oxide is highly hazardous and has a high toxicity, and thus must be carefully handled. Accordingly, an organic electronic device satisfying the desired requirements had never been obtained.
Conventional electrophotographic apparatus such as plain paper copying machine (PPC), laser printer, LED printer and liquid printer operate to apply an imaging process which comprises charging, exposure and development to a photoreceptor such as rotary drum type photoreceptor to form an image, which image is transferred to and then fixed on a transfer material to obtain a duplicated matter. Examples of the photoreceptor used in these image forming apparatus include an inorganic photoreceptor such as selenium, arsenic-selenium, cadmium sulfate, zinc oxide and a-Si or an organic photoreceptor (OPC). Among these photoreceptors, organic photoreceptors are often used because it is inexpensive and has good productivity and disposability. Among these organic photoreceptors, a functionally-separated laminated photoreceptor comprising a laminate of a charge-generating layer and a charge-transporting layer is excellent in electrophotographic properties such as sensitivity, chargeability and repetition stability. Various proposals have been made for such a functionally-separated laminated photoreceptor and put into practical use.
A corona-charging apparatus comprising a fine wire electrode such as gold-plated tungsten wire and a shield plate as main constituents has been widely used as the charging apparatus for charging such a photoreceptor.
However, such a corona-charging apparatus has the following disadvantages:
1) In order to obtain a surface potential of from 500 to 700 V on a latent image retaining member, it is necessary that a high D.C. voltage of not less than 4 kV be applied to the wire electrode. This requires that the distance between the wire electrode and the shield plate be kept great to inhibit the leakage of voltage to the shield plate or the main body. Accordingly, a large-sized apparatus is required. Further, the use of high-tension cable is indispensable. This increases the cost.
2) The corona discharge is accompanied by the production of a relatively large amount of ozone or nitrogen oxides. The resulting nitrogen oxides then react with moisture in the air to produce discharge products such as nitric acid. These discharge products are then attached to or act on the surface of the photoreceptor to modify or deteriorate the photoreceptor, causing image defects such as blurred image. These discharge products are also undesirable from the standpoint of environmental issue, which has recently been getting popular. Accordingly, the use of an exhaust fan, filter or other apparatus for removing these discharge products is indispensable, resulting in a further cost increase.
Instead of using such a troublesome corona charging apparatus, various contact-charging processes have recently been proposed. In these contact-charging processes, an electrically-conductive member to which a voltage has been applied is brought into contact with the surface of a photoreceptor so that electric charge is directly injected into the surface of the photoreceptor to obtain a desired charged potential, as described in JP-A-63-149669 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, application of such a contact-charging process to the conventional functionally-separated organic photoreceptor has the following disadvantages. (1) In general, a charge-transporting layer comprising a polymer binder resin and a low molecular weight charge-transporting material molecularly dispersed therein is used as the outermost layer. This photoreceptor is repeatedly used while the charge-transporting layer is kept in direct contact with the charging member. Thus, the charge-transporting layer remarkably wear out, thereby causing chargeability drop, sensitivity change, etc. Accordingly, the photoreceptor exhibits an extremely reduced life as compared with the case where the corona charging process is employed. (2) Since the charging member is brought into direct contact with the photoreceptor, foreign substances can easily adhere to or can contaminate the surface of the photoreceptor, causing image defects on copied image.
Various causes can be considered for the abrasion of the outermost layer of the photoreceptor and the adhesion of foreign substances to the surface of the photoreceptor. In a charge-transporting layer having a low molecular weight charge-transporting material dispersed in a binder resin, the variation of the contacting manner with the charging member causes local direct passage of current, stressing the photoreceptor not only on the surface thereof but also to the interior thereof. In a process where a d.c. current having an a.c. current superposed thereon is used, the deterioration of the charge-transporting material and the binder resin is accelerated to a greater depth. Further, if the charge-transporting layer has a locally ununiform dispersion of charge-transporting material, the above described deterioration occur ununiformly, too. It is considered that this deteriorates the strength of the outermost layer, thereby rendering the photoreceptor more apt to abrasion. At the same time, the ununiform deterioration causes the formation of nuclei attracting foreign substances.
The present invention has been achieved to solve the above described problems in conventional techniques.